The present invention concerns a process and apparatus for producing an epitaxial and/or highly textured grown film, free of foreign phases, of a high-T.sub.c (critical temperature)-oxide superconductor on a substrate.
An epitaxial and/or highly textured growth of a superconducting film on a substrate occurs when (i) the lattice constant of the substrate plane and the superconducting phase of the material are comparable, (ii) the substrate temperature corresponds to the growth temperature of the superconducting phase, (iii) the growth speed of the film permits an epitaxial orientation, and (iv) stoichiometric deviations are limited.
Remarkable success in the production of high-current carrying films of oxide superconductors can be achieved by the use of UV-EXIMER lasers. In such a process, a laser beam with a precisely adjusted energy density, generates an ablation process on a spender target comprised of a bulk material having a superconducting composition, to almost completely evaporate from the spender target a coating of a few nanometers which is deposited on a receiver target (substrate).
In general, the known oxide superconductor material are extremely unstable in the vapor phase and decay into their individual components. In spite of this, one observes in laser ablation on the receiver target a good to very good stoichiometry. Such a process is known from the publication by T. Venkatesan, C. C. Chang, D. Dijkkamp and others, Bell Communication Research, Red Bank, New Jersey, entitled "Substrate Effects on the Properties of Y-Ba-Cu-O Superconducting Films Prepared by Laser Deposition".
As can be seen from the article by S. Komuro, Y. Aoyagi, T. Morikawa and S. Namba, Japanese Journal of Applied Physics, Volume 27, No. 1, January 1988, pages L34 to L36, entitled "Preparation of High-T.sub.c Superconducting Films by Q-Switched YAG Laser Sputtering", film formation on a substrate is said to occur according to a cluster model in which the bulk material of the spender target is subjected to laser irradiation and thereby released from the spender target in the form of clusters (droplets), and these clusters then deposit on the substrate to form a film. As can be seen from the article, the as-deposited film which is formed from the clusters as a result of the droplet of the superconductor material hitting the substrate is insulating and requires a high temperature post-anneal at 900.degree. C. to make the film superconducting. At this temperature there are already very strong reactions between film and substrate, which can be considered disadvantageous because they exclude a large number of interesting substrate materials (ZrO.sub.2, Si, Pt, Al.sub.2 O.sub.3 etc. ). For this reason, the ablation process using UV-EXIMER-Lasers is carried out in such a way that the superconducting material evaporates practically completely and the film is produced on the substrate by the precipitation of discrete molecules.
The formation of droplets can be observed particularly frequently when the energy density of the laser beam is adjusted too high or when, instead of the EXIMER laser, the less expensive long-wave lasers, e.g., CO.sub.2 or argon lasers, are used.
The known process has the disadvantage that, due to the precipitation of discrete molecules on the substrate, the build-up of a superconducting film proceeds very slowly.
The formation of a superconducting film of about 300 nm requires a time elapse of about one hour according to the known process.
EXIMER lasers are very costly and thus expensive. Their degree of effectiveness is low. Due to the use of hydrogen fluoride, special safety measures have to be taken. The films produced are often not smooth, but have protrusions which cannot contribute to the current transport.